1. Field
The present invention relates to a method, servo channel, and tape drive for determining a reference waveform used by a correlator in a servo channel.
2. Description of the Related Art
In timing-based servo (TBS) systems, recorded servo patterns consist of magnetic transitions with two different azimuthal slopes. Head position is derived from the relative timing of pulses, or dibits, generated by a narrow head reading the servo patterns. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). Traditionally, the detection of LPOS information bits is based on the observation of the arrival times of the shifted dibit peaks within the servo bursts at the servo reader output. A specification for the servo format in current tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 5 (LTO-2 to LTO-5), where the servo format was not modified, can be found on the World Wide Web (www) at ultrium.com.
The timing-based servo (TBS) technology, which was developed specifically for linear tape drives and is also used in all LTO tape drive products, provides the basic structure of a servo frame, consisting of four servo bursts, as shown in FIG. 1. The signal obtained by reading the servo pattern is used to extract essential servo-channel parameters such as tape velocity, read head transversal (y)-position information, and longitudinal position (LPOS) information, which is encoded by using pulse-position modulation (PPM) with a modulation depth of ±0.25 μm in LTO drives, as also shown in FIG. 1.
The servo frame of FIG. 1 has transitions on tape with an azimuth angle of 6 degrees. Each stripe is translated by a servo reader into a pulse called dibit, which exhibits a positive peak and a negative peak. The four A, B, C, and D bursts include from left-to-right a sequence of 5-5-4-4 dibits. The arrival times of the dibit peaks in the servo bursts are also used to determine the transversal position of the reader. The frequency at which the bursts appear can be used to determine the velocity of tape. One may also decode bits encoded in the second and fourth dibits in the A and B bursts to reconstruct the longitudinal position.
In legacy timing-based servo (TBS) systems, the estimates of the servo reader lateral position, the tape velocity, and the longitudinal position (LPOS) information are directly obtained by monitoring the peak arrival times of the dibits of the servo bursts. The filtering for the servo reader signal used for the computation of the estimates is normally achieved by an anti-aliasing low-pass filter in the analog domain, prior to analog-to-digital conversion. At low tape velocities, however, a fixed low-pass filter exhibits significant excess bandwidth, which leads to excess noise level, and may greatly reduce the reliability of the computed estimates, depending on the tape velocity.
To mitigate the excess noise problem, the clock frequency of the analog-to-digital converter sampling the servo reader signal may be selected from a finite set of frequencies, which are generated by a phase-lock loop. This approach has the disadvantage that the bandwidth of the anti-aliasing filter must be selectable. A further disadvantage is that the bandwidth of the servo reader signal for the minimum cruise velocity of the tape is typically significantly smaller than the minimum available clock frequency of the analog-to-digital converter (ADC). Therefore excess noise bandwidth cannot be eliminated fully at low tape velocities. Furthermore, in legacy TBS systems the detection of LPOS information symbols, which are encoded in the servo bursts using pulse-position modulation, is performed by measuring the distances between peak arrival times and making decisions based on the measured distances, a suboptimal method that does not consider the energy of the modulated dibits.
In a current synchronous servo channel architecture, as shown in FIG. 2, the excess-noise problem in a servo reader signal with bandwidth that depends on tape velocity is solved without resorting to a low-pass filter with variable cutoff frequency. The need for a low-pass filter with variable bandwidth is avoided by introducing a matched filter interpolator/correlator, which is included in the synchronous servo channel prior to lateral position-error signal (PES) estimation, tape velocity estimation, optimum matched-filter LPOS symbol detection, and signal-to-distortion ratio estimation, as shown in FIG. 2.
FIG. 2 illustrates an architecture of a servo channel 2 according to the prior art, where an anti-aliasing filter 4 receives input from a servo reader (not shown). The output of the anti-aliasing filter 4 is routed to an analog-to-digital converter (ADC) 6 in the servo channel 2. The servo signal samples output by the ADC 6 are input to a servo channel signal interpolator 8 (also referred to as an “interpolator”) and a monitoring and control component 10. The output of the monitoring and control component 10 is routed to a time-base generator 12, to the matched-filter correlator 16 and to a peak-arrival time component 14.
The matched-filter interpolator/correlator 16 ensures that optimum filtering of the servo reader signal is performed not only at constant tape velocity, but also during acceleration and deceleration. Optimum signal filtering is thus achieved for the computation of the various estimates during all modes of operation. Proper matched-filter operation in the synchronous servo channel, however, requires the knowledge of the dibit response obtained from the servo reader. The dibit waveform depends on the definition of parameters for the servo patterns as defined, e.g., in the LTO specification, which are azimuth angle a (a=6 deg. for LTO 1 to 5), servo stripe width t (minimum distance between magnetic transitions, t=2.1 μm for LTO 1 to 5), and servo reader width. In the current synchronous servo channel implementation, matched filtering is normally performed assuming a standard reference waveform with nominal servo format parameters and given servo reader width. The current synchronous servo channel implementation is very robust for a wide range of system parameters. However, if large mismatches with respect to the LTO specification occur due to badly formatted tape cartridges or otherwise degraded operating conditions, this might lead to severe degradation in the quality of PES estimation and LPOS detection.
There is a need in the art for improved techniques for determining improved dibit reference waveforms that provide better match to the read servo signals in a synchronous servo channel.